1. Field of the Invention
The present invention relates to a laminated common-mode choke coil applied to a transmission line of high-frequency signals.
2. Description of the Related Art
For example, with a high-speed interface such as the USB (Universal Serial Bus) and the HDMI (High Definition Multimedia Interface), “the differential transmission scheme” in which signals that are 180° out of phase with each other are respectively transmitted through a pair of signal lines (=balanced line) is used. Since radiated noise and external noise are canceled out at the balanced line with the differential transmission scheme, such kinds of noises are less influential. However, practically speaking, particularly with a signal line for a high-speed interface, common-mode noise current attributed to asymmetry of the signal line occurs. Accordingly, in order to suppress the common-mode noise, a common-mode choke coil is used for a high-speed interface-purpose signal line.
Normally, as disclosed in FIG. 1 of JP 2003-068528 A and FIG. 2 of JP 2008-098625 A, the common-mode choke coil is defined as a small laminated chip component which has two coils (a primary coil and a secondary coil) wound in the identical direction. Here, the primary coil and the secondary coil are aligned in the laminated direction in a lamination element.
FIG. 13 is a cross-sectional view of a common-mode choke coil disclosed in JP 2003-068528 A. This common-mode choke coil is defined to have two coils (laminated coils) 2 and 3 in a lamination element 1. The coils 2 and 3 are wound coaxially and spaced away from each other in the axial direction. The leading ends and trailing ends of the coils 2 and 3 are lead out at the end surfaces on the opposite sides of the lamination element 1, and connected to prescribed external electrodes.
However, it is difficult to increase the degree of coupling between the primary coil and the secondary coil just by aligning the primary coil and the secondary coil simply in the laminated direction in the lamination element. When the degree of coupling between the primary coil and the secondary coil is low, the insertion loss of the normal-mode signals is disadvantageously increased. On the other hand, when the primary coil and the secondary coil are closely disposed to each other in order to increase the degree of coupling, the capacitance occurring between the primary coil and the secondary coil (stray capacitance) is disadvantageously increased. When this capacitance becomes great, the differential impedance of the common-mode choke coil is reduced, thus failing to match with the impedance of the balanced line.
Further, with the structure in which the primary coil and the secondary coil are aligned in the laminated direction in the lamination element, misalignment of the formation position of coil patterns or misalignment in laminating the sheets occurs as a problem in the process. Further, when the common-mode choke is mounted on a printed circuit board, the balance between the capacitance between the primary coil and the ground conductor and the capacitance between the secondary coil and the ground conductor is lost because of problems in the structure such as a difference between the coils in the coupling amount with the ground conductor on the printed circuit board. This makes it impossible to secure symmetry between the primary coil and the secondary coil, and common-mode noise is converted into normal-mode signals. That is, the capability of removing common-mode noise is disadvantageously reduced.
Further, in some cases, a magnetic substance is used as the lamination element. However, since the magnetic substance has relatively great frequency dependence, the loss of normal-mode signals particularly at high frequency bands tends to become great. Further, a full coupling value cannot be obtained between the primary coil and the secondary coil particularly at high frequency bands, and the loss of normal-mode signals tends to become great.